1. Field of the Invention
The magnetic memory according to the present invention is applicable in wide technical fields such as a magnetic memory, magnetic sensor and spin electronic device. The present invention particularly provides a useful device as a part of the solid magnetic memory device.
2. Description of the Related Art
A DRAM, SRAM, flash memory, EEPROM and FeRAM have been used as the memory device in recent years. However, the solid magnetic memories, in particular memories taking advantage of TMR and GMR effects, have became great matters of interest from the view point of non-volatility, high speed and high density of memories, and studies of them are in progress. The magnetic memory closely related to the present invention will be described hereinafter.
The giant magnetoresistance (GMR) will be described at first. GMR was discovered by Fert and Grunberg in 1986 to 1988. They found that a gigantic magnetoresistance larger than AMR is observed in a ferromagnetic (Fe)/non-magnetic (Cr) artificial lattice, and they named this phenomenon as the giant magnetoresistance (GMR). This GMR has a negative magnetoresistivity change against an applied magnetic field with a magnetoresistivity change of as large as several scores of percentages. The cause of GMR is qualitatively elucidated as follows. When no magnetic field is applied, magnetic layers in the artificial lattice are aligned in an antiferromagnetic relation with each other (interlayer antiferromagnetism). When a magnetic field is applied to these layers, magnetization of each layer is aligned to be parallel with each other. Then, electric resistance is decreased by the magnetic field under a spin-dependent mechanism by which conduction electrons having antiparallel spins to the direction of magnetization are strongly scattered while the electrons having parallel spins to the direction of magnetization of magnetization are weakly scattered. The antiparallel relation of magnetization between the layers have been theoretically studied based on RKKY type long distance exchange interaction and quantum well model, and spin dependent scattering between the layers has been discussed by a theory based on a two liquid model of the conduction electrons.
For utilizing this GMR effect in memory devices, for example, the direction of magnetization of a part of the ferromagnetic layers is fixed, and the direction of magnetization of the remaining ferromagnetic layers are changed to permit the ferromagnetic layers to serve as a memory. The devise so constructed as described above is named as a spin-valve type thin film magnetic element. The layer in which the direction of magnetization remains unchanged (or a layer having a high coercive force) is named as a pinned or hard (ferromagnetic) layer, and the layer in which the direction of magnetization varies (or a layer having a low coercive force) is named as a free (ferromagnetic) layer. In a different method, on the contrary, a line of magnetic information is recorded on the hard layer and magnetization states (memory states) are read from magnetoresistivity changes caused by reversing magnetization of the free ferromagnetic layer.
The types of GMR known in the art include a CIP type, a CPP type, a CAP type as a mixed type of the CIP and CPP types, and a granular alloy type. Generally speaking, the CIP structure has been most frequently studied because of its easiness in manufacturing. However, the magnetoresistivity change is about 40 to 50% in the CIP type GMR in which an electric current flows parallel to the lamination face, due to contributions of the conduction electrons that are not involved in spin scattering at the interface. In the CPP type GMR in which the electric current vertically flows against the lamination face, on the other hand, the magnetoresistivity change often exceeds 100% since all the electrons experience spin scattering at the interface of the lamination layers depending on their spin states, and due to increased Fermi velocity ascribed to an energy gap arising from the lamination structure. Therefore, the CPP type is superior in basic characteristics to the other type.
However, since the electric current vertically flows against film faces in the CPP type, the magnetoresistance of the film tends to be small. Consequently, micropores involved in the laminated structure should have a quite fine cross sectional area.
An examples of the structure that is not a simple lamination structure of the ferromagnetic layer/non-magnetic layer formed in the micropore of the CPP type GMR device is described in Applied Physics Letters, Vol. 70, 396 (1997). This report describes a structure in which a three layer structure of NiFe alloy/Cu/NiFe alloy is inserted between two thick Cu layers, and an effect for decreasing a saturation magnetic field is expressed by the action of this layered structure. However, no memory effect is manifested in this example.
An example of expression of the memory effect is described in Applied Physics Letters, Vol. 76, 354 (2000). In this paper, a laminated structure is formed by inserting a thick ferromagnetic layer (a hard ferromagnetic layer 15) and a thin ferromagnetic layer (a free ferromagnetic layer 14) are inserted between the two non-magnetic layers 16 as shown in FIG. 6A, thereby succeeding in obtaining a memory effect comprising about 10% of magnetoresistivity changes. However, inversion of the memory states is not clearly shown.
Conventionally available memory cell utilizing a tunnel junction is disclosed in U.S. Pat. No. 5,764,567. Such cell usually has a laminated structure comprising an antiferromagnetic layer 63, a pinned layer 61, a non-magnetic layer 62 (an insulation layer) and a free ferromagnetic layer 14 as shown in FIG. 6B. The direction of magnetization in the ferromagnetic layer is usually in the longitudinal axis of the cell. The tunnel electric current particularly increases when the two ferromagnetic layers separated with the insulation layer have the same direction of magnetization with each other, resulting in a small electric resistance of the cell. When the two ferromagnetic layers separated with the insulation layer have different directions of magnetization with each other, on the contrary, the tunnel electric current is reduced to consequently increase the electric resistance of the cell. As shown in FIG. 6B, the direction of magnetization of the ferromagnetic layer is usually fixed in the ferromagnetic layer (pinned layer 61) as one of the two ferromagnetic layers, and the direction of magnetization of the other ferromagnetic layer (free ferromagnetic layer 14) is made to be variable. The direction of magnetization of this free ferromagnetic layer 14 is controlled by the magnetic field generated by the electric current flowing in write lines disposed on and under the cell. In other words, signals are written on only selected cell portions by a vector sum of the magnetic field from the upper and lower wiring lines. The signal is read by upper and lower read lines formed in the cell. Cell is selected using a MOSFET.
While a larger magnetoresistivity change is arithmetically possible, the corresponding value of the practically available device is in the range of 40 to 60%. The method for manufacturing the insulation layer and bias dependency of the magnetoresistivity change are the most crucial problems in manufacturing the device and on the characteristics of the device. An insulation layer with a uniform thickness of about 1 nm required for the device is difficult to manufacture. Moreover, it is an problem that the magnetoresistivity change is largely decreased when the voltage is increased due to bias dependency. These problems have not been encountered in the GMR device.
Since the CPP type GMR structure is used in the present invention, micropores having a large aspect ratio are required. For obtaining such structure, a membrane filter manufactured by a track etching method or an anodic oxidation alumina are used. The most preferable anodic oxidation alumina will be described in detail hereinafter.
An anodic oxidation alumina layer as a porous anodic oxidation film is formed by anodic oxidation of an Al plate in an acidic electrolyte solution such as a solution of sulfuric acid, oxalic acid and phosphoric acid[see for example R. C. Furneaux, W. R. Rigby and A. P. Davidson, NATURE Vol. 337, p147 (1989)]. The porous film has a peculiar geometric structure in which cylindrical micropores (nano-holes) as fine as several nanometers to several hundreds of nanometers in their diameter are arranged in parallel with spaces of several scores of nanometers to several hundreds of nanometers among them. These cylindrical micropores have a high aspect ratio as well as uniform cross sectional diameter.
The structure of the porous film may be altered to a certain extent by changing the conditions of anodic oxidation. For example, the interval between the micropores, the depth of the micropores and the diameter of the micropores are controllable by changing the anodic oxidation voltage, by controlling the anodic oxidation time and by applying a pore-widening treatment, respectively, in a certain extent. The pore-widening treatment as used herein refers to an etching treatment of alumna that is usually applied by wet etching using phosphoric acid.
A method for improving vertical orientation, linearity and isolation of the porous film by applying two step anodic oxidation, or a method for forming the porous film comprising micropores with better vertical orientation, linearity and isolation by repeatedly applying anodic oxidation after once removing the porous film formed by applying anodic oxidation, have been proposed (Japanese Journal of Applied Physics, Vol. 35, Part 2, pp. L126-L129, Jan., 15, 1996). This method takes advantage of the facts that depressions on the Al plate formed by removing the anodic oxidation film formed by the first anodic oxidation function as initiation points for forming the micropore in the second anodic oxidation.
Another method for forming the porous film having the micropores with improved shapes, spaces and patterns has been proposed [for example, Japanese Patent Laid-open No. 10-121292, or Masuda, Kotai Butsuri (Solid State Physics) 31, 493 (1996)], wherein a stamper is used for forming the micropore initiation points, or anodic oxidation is applied after forming the depressions, generated by pressing a substrate comprising a plurality of projections on the surface onto the surface of the Al plate, as the micropore initiation points. In a different art reported, the micropores form a concentric circle instead of forming a honeycomb structure (for example, Japanese Patent laid-open No. 11-224422).
Japanese Patent Laid-open No. 10-283618 discloses an art in which a laminated magnetic film having GMR characteristics is embedded in the nano-hole formed by anodic oxidation. Although this patent publication describes an art in which the hard ferromagnetic layer and free ferromagnetic layer are formed by composition differences between the layers, the magnetoresistivity change is about 10% while manifesting an insufficient memory effect.
The conventional GMR device as hitherto described is a CIP type device, and has an insufficient magnetoresistivity change of about 40%. Stability and uniformity of the CCP type GMR device was also insufficient.
Accordingly, it is an object of the present invention for solving the problems in the related art to provide a high density magnetic device and a method for readily manufacturing the magnetic device, and to provide a solid magnetic memory comprising the magnetic device, wherein the CPP type GMR device comprises a hard layer excellent in stability in a cell having a large aspect ratio, and a laminated structure by which initial states of magnetization of the hard layer and free ferromagnetic layer can be easily generated with good stability.
The present invention for attaining the foregoing objects provides a magnetic device to be used by flowing an electric current in the direction of depth of pores. The magnetic device comprises a porous layer on a substrate, a first ferromagnetic layer, and a second ferromagnetic layer having a smaller coercive force than the first ferromagnetic layer. The ferromagnetic layers and a non-magnetic layer are laminated within all or a part of pores. The first ferromagnetic layer is laminated on the second ferromagnetic layer by being separated with the non-magnetic layer. The first ferromagnetic layer contains the same elements as in the second ferromagnetic layer, and the proportion of each element is different between the two ferromagnetic layers.
The present invention also provides a method for manufacturing the magnetic device, wherein the hard ferromagnetic layers comprising the same elements with different proportions between the layers, and the free ferromagnetic magnetic layer are formed by electrodeposition in the same electrolyte solution by changing electrodeposition potentials among the steps for electrodepositing the hard ferromagnetic layers and free ferromagnetic layer.
Preferably, the non-magnetic layer mainly comprises Cu, and the ferromagnetic layers mainly comprise Co or a FeNi alloy. The magnetic device is preferably formed by laminating ten cycles or more of one unit comprising a laminated layer of (hard ferromagnetic layer/non-magnetic layer/free ferromagnetic layer/non-magnetic layer) in which the hard ferromagnetic layer and free ferromagnetic layer are laminated by being separated with the non-magnetic layer in order to secure a sufficient magnetoresistance.
Preferably, the magnetic device comprises a micro-porous layer comprising alumina nano-holes formed by anodic oxidation. Preferably, an underlayer mainly comprising Cu is formed at the bottom of the micro-porous layer, and the plural micropores are arranged in a honeycomb shape or rectangular shape.
Further objects, featured and advantages of the present invention will become apparent from the following descriptions of the preferred embodiments with reference to the attached drawings.